1. Field of the Invention
The present invention relates to a method for improving a characteristic of a dielectric material. More particularly, the present invention relates to a method for improving a characteristic of a dielectric material by hydrogen plasma.
2. Description of the Related Art
As the integration density of semiconductor devices increases, more circuit elements must be packed into a unit surface area of the device substrate, and circuit elements such as interconnects are necessarily increased between MOS transistors of the IC device. In many highly integrated semiconductor devices, more than two levels of interconnecting metal layers are necessary, called multilevel interconnects. Between these multiple metal layers, electrically insulating material known as inter-metal dielectrics are used to provide isolation between the metal layers.
In the era of deep sub-micron semiconductor fabrication, the need to minimize interconnect capacitance is becoming more important to overcome the increased RC delay which results from shrinking geometry. One method of minimizing the capacitance of the interconnect structure of a semiconductor device is to provide a dielectric material having a low dielectric constant (K) between the conductive traces of the interconnect structure.
The shrinkage of feature size in such integrated circuit structures includes reduced horizontal spacing between adjacent conductors on the same plane. However, such decrease of feature size results in a corresponding rise in the impedance of the conductors, as well as crosstalk between the conductors. Such increases in impedance in the integrated circuit structure can result in degradation of the performance of the integrated circuit structure.
In order to decrease RC delay, power and crosstalk, a methylsilsesquioxane (MSQ) film having a low dielectric constant about 2.6-2.7 is usually applied in the process of fabricating a semiconductor device. The methylsilsesquioxane film is usually formed by spin coating, so that the methylsilsesquioxane film is smooth enough. Therefore, the methylsilsesquioxane is widely applied.
Conventionally, a photoresist layer is removed by an ashing method using oxygen plasma. Removing the photoresist layer is an important part of the process of fabricating a semiconductor device. Since the semiconductor device is very small as compared to a large scale integrated circuit device (LSI) or a very large scale integrated circuit device (VLSI), the photoresist layer is hard to remove, by the usually ashing method using the oxygen plasma, without damaging the devices.
Generally, the photoresist layer is not easily etched as intended by the ashing method. In other words, the photoresist layer is hard to etch precisely so as to be able to remove only the photoresist layer without damaging any of the devices in the ashing process. Additionally, the leakage current density is increased by removal of a photoresist layer by the ashing method using the oxygen plasma, and the dielectric constant is also increased.
FIG. 1A is a network structure for methylsilsesquioxane. Referring to FIG. 1A, methylsilsesquioxane comprises silicon atoms 12, oxygen atoms 14 and methyl groups 10, wherein the silicon atoms 12 and the oxygen atoms 14 make up a network structure, and the methyl group 10 links with the silicon atom 12.
FIG. 1B is a per unit polymer of methylsilsesquioxane. Referring to FIG. 1B, a per unit polymer of methylsilsesquioxane is made up many monomers. Each of the monomers comprises silicon atoms 12, oxygen atoms 14 and methyl groups 10, wherein the silicon atoms 12 and the oxygen atoms 14 makes up a network structure, and the methyl group 10 links with the silicon atom 12.
FIG. 2 shows the leakage current density of methylsilsesquioxane after oxygen plasma treatment as a function of electric field.
Referring to FIG. 2, curves 20, 22, 24 and 26 represent the leakage current density of methylsilsesquioxane without oxygen plasma treatment, and the methylsilsesquioxane with oxygen plasma treatment for 3 min, 6 min and 9 min as a function of an electric field, respectively. As can be seen from FIG. 2, the curve 20 is the lowest, which means that leakage current of the methylsilsesquioxane without oxygen plasma treatment is lower than that of the methylsilsesquioxane with oxygen plasma treatment. The leakage current of the methylsilsesquioxane increases as oxygen plasma treatment time is increased. In other words, the oxygen plasma destroys the characteristically low dielectric constant of the methylsilsesquioxane. As a result, the dielectric constant of methylsilsesquioxane increases as oxygen plasma treatment time increases.